This invention relates generally to spintronics. In particular, the invention relates generally to efficient room temperature spin injection into semiconductors.
Over the past decade a pursuit of solid state ultrafast scaleable devices based on both the charge and spin of an electron has led to a development of new fields of magnetoelectronics and spintronics. The discovery of giant magnetoresistance (GMR) in magnetic multilayers has quickly led to important applications in storage technology. GMR is a phemonenon where a relatively small change in magnetism results in a large change in the resistance of the material.
The phenomenon of a large tunnel magnetoresistance (TMR) of ferromagnet-insulator-ferromagnet structures is a focus of product development teams in many leading companies. TMR is typically observed in F1-I-F2 structures made of two ferromagnetic layers, F1 and F2, of similar or different materials separated by the insulating thin tunnel barrier I with thickness typically ranging between 1.4-2 nm. The tunnel current through the structure may differ significantly depending on whether the magnetic moments are parallel (low resistance) or anti parallel (high resistance). For example, in ferromagnets such as Ni80Fe20, Coxe2x80x94Fe, and the like, resistance may differ by up to 50% at room temperature for parallel (low resistance) versus antiparallel (high resistance) moments on ferromagnetic electrodes.
It is worth mentioning recent studies of the giant ballistic magnetoresistance of Ni nanocontacts. Ballistic magnetoresistance is observed in Ni and some other nanowires where the typical cross-section the nano-contacts of the nanowire is a few square nanometers. The transport in this case is through very short constriction and it is believed to be with conservation of electron momentum (ballistic transport). The change in the contact resistance can exceed 10 fold (or over 1000%).
Of particular interest has been the injection of spin-polarized carriers, which are currents mainly in the form of spin-polarized electrons into semiconductors. This is largely due to relatively large spin-coherence lifetimes of electrons in semiconductors, possibilities for use in ultra fast devices, use in quantum communication and computation and the like. The possibility of spin injection from ferromagnetic semiconductors (FMS) into nonmagnetic semiconductors has been demonstrated in a number of recent publications. However, the Curie temperature (the temperature above which a material becomes non-magnetic) of magnetic semiconductors is substantially below room temperature. The low Curie temperature limits possible applications. Room-temperature spin injection from ferromagnets (FM) into semiconductors also has been demonstrated, but it remains a difficult task and the efficiency is very low.
The main problem of the spin injection from a ferromagnet into a semiconductor is that a potential barrier (Schottky barrier) always arises in the semiconductor near the metal-semiconductor interface. Numerous experiments show that the barrier height xcex94 is determined by surface states forming on the interface, and is approximately ( ) Eg practically independently of type of the metal, where Eg is the energy band gap of the semiconductor, i.e. the difference between the conduction band energy level EC and the valence band energy level EV. For example, barrier height xcex94 is about 0.8-1.0 eV for GaAs and about 0.6-0.8 eV for Si.
FIG. 1A illustrates a schematic of a conventional spin-injection device 100. As shown, the spin-injection device 100 includes a semiconductor 110 and a ferromagnetic (FM) layer 120 above the semiconductor 110. The device 100 also includes electrodes 130 and 140 connecting to the ferromagnetic layer 120 and the semiconductor 110. As will be shown below, the Schottky barrier formed in such a device is very wide, which makes tunneling of electrons practically impossible.
FIG. 1B illustrates an energy band diagram of the conventional spin-injection device 100 illustrated in FIG. 1A. The barrier for electrons has a height xcex94 and width l (which is the width of the Schottky depletion layer). The electrons with energy E should tunnel a distance x0 of the barrier. As noted above, the height xcex94≈⅔ Eg, practically independently of type of the metal.
The amount of spin injection is determined by current in the reverse direction through the Schottky barrier. This current is usually extremely small mainly due to the relatively large barrier width l (e.g. width l greater than 40 nm in not too heavily doped semiconductors with Nsxe2x89xa61018 cmxe2x88x923, where Ns is the donor concentration level of the semiconductor) and the barrier height xcex94, which is much greater than kBT, where kB is the Boltzmann constant and T is the device temperature. The current through the structure is determined by electron thermionic emission, which is extremely small (because the barrier is high compared to the temperature, xcex94 greater than  greater than kBT). Therefore, the effective spin injection in the conventional device 100 is impossible for all practical purposes.
According to an embodiment of the present inventions, a spin-injection device comprises a ferromagnetic layer, a semiconductor, and an extremely thin and extremely heavily doped layer, xcex4-doped layer, between the ferromagnetic layer and the semiconductor.